Current gain filter

ABSTRACT

A current gain filter including an electrical circuit containing a number of passive elements which divide the current flow from a current generator into several current components whose vector sum is greater than the input current from the current generator. In one form of the invention, resistive and capacitive elements are utilized; in a second form, resistive and inductive elements are utilized; and in a third form, the circuit contains resistive, capacitive and inductive elements. For each form of the invention, a transfer function can be derived which contains several independent parameters by means of which the ratio of output current to input current and the phase angle therebetween can be calculated.

United States Patent [19] Murphy [111 3,818,387 June 18, 1974 CURRENT GAIN FILTER [76] Inventor: James J. Murphy, 2070 Latham,

Apt. No. 4, Mountain View, Calif. 94040 [22] Filed: Aug. 2, 1972 21 Appl. No.; 277,214

[52] US. Cl. 333/70 R, 333/76 [5 1] Int. Cl. H03h 7/04 [58] Field of Search 333/70 R, 76

[56] References Cited UNITED STATES PATENTS 7/l966 White 333/70 R OTHER PUBLICATIONS Hill New York 1953; pages l32138.

Primary Examiner-Archie R. Borchelt Assistant Eramine rMarvin Nussbaum [57] ABSTRACT A current gain filter including an electrical circuit containing a numberof passive elements which divide the current flow from a current generator into several current components whose vector sum is greater than the input current from the current generator. In one form of the invention, resistive and capacitive elements are utilized; in a second form, resistive and inductive elements are utilized; and in a third form, the circuit contains resistive, capacitive and inductive elements. For each form of the invention, a transfer function can be derived which contains several independent parameters by means of which the ratio of output current to input current and the phase angle therebetween can be calculated.

10 Claims, 9 Drawing Figures PATENTED 1 8 I974 satcunrs FIG! PAIENTED JUN 1 8 m4 PHASE ANGLE(DEGREES) 1 CURRENT GAIN FILTER manner to produce a current gain without the benefit of any amplifying device in the circuit. This is achieved by the arrangement of such elements in the circuit in a manner to define several current paths between a current generator and a reference level, such as ground, and to assure that the vector sum of the currents through at least certain of such paths results in an output current whose magnitude is greater than that of the input current from the current generator. This result is achieved by selecting the elements in each path-so that the currents therethrough lag or lead such input current, depending upon the magnitude and direction required to provide the desired vector sum.

In one embodiment of the invention, resistive and capacitive elements make up the components of the circuit, such elements being connected together so that the downstream element in each of a pair of current paths will have a current flow whose magnitude and direction is such that when it is added vectorially to the corresponding current flow in the other current path, the vector sum will represent the output current of the circuit with such output current having a greater magnitude but the same direction, i.e., in phase with the input current to the circuit. Such greater magnitude of the output current represents a current gain attained by the proper selection of the passive elements and their relative positions in the circuit.

In another embodiment of the invention, resistive and inductive elements make up the components of the circuit. These elements are also arranged to give the vector sum in the manner described.

In a third embodiment of the invention, the circuit is comprised of resistive, capacitive and inductive elements all arranged for the purpose described above with respect to the first and second embodiments. In each embodiment, a transfer function or equation can be derived, wherein a relationship is established between the ratio of the output and input currents, the frequency of the current from the current generator and the several independent parameters of the corresponding circuit. Solving this equation using various values for such frequency and parameters yields a family of curves by means of which optimum values of the parameters can be determined.

While voltage gain filters have been known in the past, current gain filters are generally new in the art. Voltage gain filters have been disclosed in the Australian Journal of Instrument Technology, May 1965, in an article entitled A Cathode-Follower Oscillator, and in the Sept. 16, 1968, issue of Electronics, in an article entitled Parallel-T Bandpass Filter Produces Voltage Gain." Neither disclosure alluded to current gain as only voltage gain is attainable by the circuits thereof. As the present invention makes evident, voltage gain filter circuits are completely different from each embodiment of the present invention due principally to the fact that the current gain filter of the invention provides a combination of passive elements completely different from the combinations of elements in the voltage gain filters of the above disclosures.

The primary object of this invention, therefore, is to provide a current gain filter comprised of passive circuit elements which cooperate in a manner to provide a gain in the output current of the circuit over the input current thereof without the use of amplifier devices wherein such gain is achieved by the specific selection and arrangements of the elements so that the currents therethrough either lag or lead the input current to assure that the vector sums of certain of the currents yields the output current whose magnitude is greater than the input current.

Another object of this invention is to provide a current gain filter of the type described wherein the passive elements of the circuit can be either resistive and capacitive components, resistive and inductive components or resistive, capacitive and inductive components, so that the currents through the various elements, when properly selected and arranged, can be caused to lag or lead the input current to assure that the currents which are combined to yield the output current will have a vector sum whose magnitude exceeds that of the input current of the circuit and will have respective directions causing the output current to have a predetermined phase with respect to said input current.

Other objects of this invention will become apparent as the following specification progresses, reference being had to the accompanying drawings for an illustration of the invention.

IN THE DRAWINGS:

FIG. 1 is a schematic view of a first embodiment of the current gain filter of this invention wherein resistive and capacitive elements are used in the filter circuit;

FIG. 2 is an equivalent circuit of the current gain filter of FIG. 1;

FIG. 3 is a vector diagram showing the individual current components through the elements of the circuit of FIG. 1 and illustrating the current gain resulting from the use of the circuit;

FIG. 4 is a graphical representation of the current gain as a function of frequency for various values of the independent circuit parameters;

FIG. 5 is a graphical representation of the phase angle between the output and input currents of the circuit of FIG. 1 plotted against frequency for the values of the independent parameters selected in determining the current gains shown in FIG. 4;

FIG. 6 is a schematic view of a second embodiment of the current gain filter of this invention, wherein resistive and inductive elements are used in the filter circuit;

FIG. 7 is a view similar to FIG. 3 but showing the vector diagram corresponding to the circuit of FIG. 6;

FIG. 8 is a schematic view of a third embodiment of the current gain filter of this invention, wherein resistive, capacitive and inductive elements are used in the filter current; and

FIG..9 is a view similar to FIGS. 3 and 7 but showing the vector diagram corresponding to the circuit of FIG.

The first embodiment of the current gain filter of this invention is broadly denoted by the numeral 10 and is shown in FIG. 1. It includes a current generator 12 in combination with a number of resistive and capacitive elements arranged to form four distinct current flow paths relative to generator 12 and relative to a predetermined reference which, for purposes of illustration, will hereinafter be referred to as ground".

Generator 12 is coupled at one side to ground and at the other side to a pair of current flow paths denoted by the numerals 13 and 15, respectively. Path 13 has a resistor 14 in series with a capacitor 16, the resistor being coupled between the capacitor and generator 12. Capacitor 16 is coupled to a lead 18 which goes to ground and represents the current flow path for the output current of circuit 10. Path 15 has a capacitor 20 in series with a resistor 22, the capacitor being between the resistor and generator 12, and the resistor being coupled to lead 18. Thus, the output current flowing to ground along lead 18 at the sum of the currents passing through capacitor 16 and resistor 22.

Another current flow path is formed by a resistor 24 coupled between ground and the common lead 26 between resistor 14 and capacitor 16. A fourth current flow path is formed by a capacitor 28 coupled between ground and lead 30 common to resistor 22 and capacitor 20.

The currents flowing through the various current flow paths of circuit 10 are best shown in the stretched out version of the circuit as shown in FIG. 2. Current I. issuing from current generator 12 is divided between paths 13 and 15, the current through resistor 14 being denoted by the numeral 1 and this latter current being divided into two components, namely, 1 flowing through capacitor 16 and I flowing through resistor 24. In path 15, the current I, through capacitor 20 is divided into two components, namely, I flowing through resistor 22 and I flowing through capacitor 28. The output current, denoted by the numeral 1 and shown in FIG. 1, is the vector sum of currents I and 1 Circuit has a characteristic transfer function associated with it which is given in terms of frequency and several independent parameters. The transfer function for circuit 10 is given as follows:

where j}, is a constant and is equal to /21rRC, in Hertz R is the resistance of resistor 22 in Ohms C is the capacitance of capacitor 16 in Farads f is the frequency of the current in Hertz b and K are independent parameters By properly selecting the values of b and K and specific values of the ratio f/f a family of curves of the type shown in FIG. 4 can be plotted. These curves show the current gain, i.e., the ratio of 1-, to 1 versus frequency for the various values of b and K. It is noted that curve A shows a gain greater than unity, the gain being approximately 1.207 when the value of K is infinity and when the value of b is 2.414. In curves B through E, the value of K becomes progressively less, from 10 to 2; whereas, the value of b becomes progressively greater, from 2.609 to 4.309. For all curves, the maximum gain occurs at the center frequency of circuit 10, i.e., when the ratio f/fl, is equal to 1. These curves, therefore, indicate that the output current of circuit 10 is greater than the input current at frequencies near the center frequency of the circuit.

In FIG. 5, the phase difference between I and I. is plotted versus frequency for the b and K values corresponding to curves A through E of FIG. 4. From FIG. 5, it can be seen that the input and output currents are substantially in phase at the center frequency but are out of phase on either side of such center frequency.

The current gain resulting from the use of circuit 10 can also be shown with reference to the vector diagram of FIG. 3 wherein the various currents in the circuit are plotted vectorially with reference to the input current I which is arbitrarily taken as unity and shown on the X-axis. This particular vector diagram corresponds to curve A of FIGS. 4 and 5. In FIG. 3, current I, lags current I by approximately 45, and I, is the vector sum of currents l and 1 Current 1 is perpendicular to current 1 and leads the same by an angle of since current I flows through a capacitor, while current 1 flows through a resistor.

Current 1.; leads current I by 45 and is the vector sum of currents I and I Current 1 is perpendicular to current 1 and leads the same by an angle of 90 because I is a current through a capacitor and I is a current through a resistor. Since the output current I, is a vector sum of currents l and 1 these are summed vectorially in FIG. 3 and as shown, their vector sum results in a vector 1 which is in phase with I but has a magnitude greater than 1 As plotted graphically, the magnitude of 1 is approximately 1.207 which is consistent with curve A of FIGS. 4 and 5.

FIG. 3 also shows that currents l and I are negative in value and that their vector sum is approximately .207. Thus, these currents actually flow upwardly in FIG. 2 rather than downwardly as shown, thereby providing the means by which current gain can be realized in the circuit. It can, therefore, be seen that, by the judicious selection and arrangement of currentcarrying components of a filter circuit wherein the currents through the components are in predetermined phase relationships to each other, the circuit can provide current gain in a very simple manner.

A second form of the invention is shown in FIG. 6 and comprises a filter circuit which is substantially the same in construction as circuit 10 except that, whereas circuit 10 has capacitors 16, 20 and 28, circuit 110 has inductors 116, 120 and 128, respectively. Thus, circuit 110 includes a current generator 112 whose output current (the input current of circuit 110) is denoted by the numeral 1,, and is directed along two paths 113 and 115, current path 113 having resistor 114 and inductor 116, and path 115 having inductor and resistor 122. Current I flowing through resistor 114 leads the input current I by a phase angle of 45 and is divided into two currents, namely, I, through inductor 116 and 1 through resistor 124 to ground. Current I through inductor 120 lags the input current 1 by a phase angle of 45 and is divided into two currents, namely, 1 flowing through resistor 122, and i flowing through inductor 128 to ground. The output current I1 is the vector sum of currents I and I Circuit 110 has the same characteristic transfer function associated with it as does circuit 10, the transfer function being given above, except that f in Hertz is equal to R/21rL where R is the resistance of resistor 122 in Ohms and L is the inductance of inductor 116 in Henries. These curves will show current gain for circuit 110 which is greater than unity. For instance, if b is given the value of 2.414 and K is at infinity. the value of the current gain, i.e., the ratio of I to I is approximately 1.207. Since the family of curves in FIG. 4 have been shown with respect to circuit 10, it is not deemed necessary to repeat this set of curves for circuit 110. Also, a set of curves corresponding to those in FIG. 5 can also be obtained to show the phase angle versus frequency for various values of b and K for circuit 110.

To illustrate the current gain achieved with the circuit of 110, reference is made to FIG. 7 which is a vector diagram of the currents through circuit 110 for the case where b is equal to 2.414 and K is at infinity. This diagram shows that current 1 leads the input current I by a phase angle of 45. It also shows that current I, is the vector sum of currents l and I the latter current leading current 1 by a phase angle of 90 because current 1 flows through an inductor and current 1 flows through a resistor.

FIG. 7 further shows that current I, flowing through inductor 120 lags the input current I by a phase angle of 45 and that current I, is the vector sum of currents I and 1 Current 1 lags current 1 by a phase angle of 90.

The output current 1 is the vector sum of currents l and I and when correctly plotted, the resultant vector 1, has a value greater than that of input current 1 and is in phase therewith. For the particular values of b and K and for the value of f/f of unity, the current gain is 1.2071.

FIG. 7 further shows that currents I and I are negative in magnitude and that their resultant or vector sum is equal to .2071. Thus, these two currents actually flow upwardly in circuit 110 when viewing FIG. 6 rather than downwardly as shown. This fact establishes that these two currents contribute significantly to the overall current gain of the circuit and are the reason why current gain is achieved by operation of circuit 110.

A third form of the invention is shown in FIG. 8 and comprises a circuit 210 having a current generator 212 coupled at one end thereof to ground and at the other end to a pair of circuit portions which divide the output current 1 of the generator (the input current to circuit 210) into two current components, namely, current I, which flows into a first current path 213 and current I, which flows into a second current path 215. Current I, flows through an inductor 214 and then is divided into two currents, namely, a current 1 through a capacitor 216 and a current I which flows through a resistor 224 to ground. Current I flows to ground by way of a lead 218.

Current I. flows through a resistor 220 and then is divided into two currents, namely, current flowing through a resistor 222 and current I flowing through a capacitor 228 to ground. Current I is coupled by lead 218 to ground and combines with current 1 to provide the output current 1 of circuit 210.

Circuit 210 has a characteristic transfer function associated with it which is given in terms of frequency and several independent parameters. The transfer function for circuit 210 is given as follows:

The frequency where the output current is in phase with the input current although not absolutely at the maximum value of the output current is given as follows:

where f is a constant and is equal to i'TTRC, Hertz. KL/C- =KR R is the resistance of resistor 222 in Ohms KL is the inductance of inductor 214 in Henries C is the capacitance of capacitor 216 in Farads f is the frequency of the current in Hertz b and K are independent parameters 5 By properly selecting the values of b and K and the specific values of the ratio f/f a family of curves of the type shown in FIG. 4 can be plotted. These curves will show the current gain, i.e., the ratio of 11 to 1 versus frequency for the various values of b and K. It is noted that the gain will be greater than unity and approximately 1.207 when the value of K is 1 and the value of b is 2.414. v

The current gain resulting from the use of circuit 210 can be shown in a vector diagram in FIG. 9 wherein the input current 1 is taken arbitrarily as'unity. Current I, lags the input current by a phase angle of approximately 29.85 and is divided into currents l and I current 1 lagging current I by a phase angle of FIG. 9 also shows that current 1 leads the input current by a phase angle of approximately 65.53 and is divided into currents l5 and 1 current 1 leading current 1 by a phase angle of 90. The output current 11 is the vector sum of currents I and 1 which, as shown in FIG. 9, is in phase with the input current 1 but greater in magnitude, the magnitude of vector 1 being approximately 1.2071, the value of the current gain for circuit 210 for the case where b is 2.414, K is unity and the frequency f is 1.5538 times the frequency 1%.

FIG. 9 further illustrates that currents I and 1 are negative in magnitude. Thus, the direction of these currents, when viewing FIG. 8, is upwardly rather than downwardly. Hence, their magnitude are added to that of current 1 thereby resulting in a current gain greater than unity.

Circuits 10, and 210 provide the current gain mentioned above because of the proper selection and arrangement of current-carrying components which cause certain of the currents through the components to lag or lead other current components in the circuit.v

By properly selecting the magnitudes of the currents and their directions relative to each other, a resultant vector representing the output current of the circuit can be arrived at wherein the magnitude of the output current is greater than that of the input current, thereby providing for the current gain for the particular circuit.

In deriving the transfer function equation for each of the foregoing circuits, reference is had to the fact that the circuit elements through which currents I and 15 of each circuit flow, for instance, capacitor 16 and resistor 22 of circuit 10, are the bases upon which the other impedances of the circuit are determined. Thus, in cirv cuit 10, the impedance Z, of resistor 22 is R, the value of its resistance, and the impedance Z, of capacitor 16 is equal to fix, The impedances of the other circuit elements of circuit 10 can be written in terms of b and K as multiplying factors of either R or X For instance, the impedance of capacitor 28, denoted by Z can be written as j b X; the impedance of capacitor 20, denoted by Z;,, can be written as j K X; the impedance of resistor 14, denoted by Z can be written as KR; and

the impedance of resistor 24, denoted by 2,, can be written as bR. By summing the currents through circuit and by using Ohms Law, the transfer function can be derived for each of the foregoing circuits in terms of b, K and the frequency f of the generator current.

Typical values for the elements for circuits 10 are as follows:

resistor 14 16,000 ohms resistor 22 1,600 ohms resistor 24 4,172 ohms capacitor 16 .1 microfarad capacitor .01 microfarad capacitor 28 .038 microfarad Using these elements, the center frequency of the circuit is 994.72 hertz or, nominally, 1,000 hertz.

For circuit 110, the values of the elements can be typically as follows:

resistor 114 8,000 ohms resistor 122 1,000 ohms resistor 124 2666.7 ohms inductor 116 10 millihenries inductor 120 80 millihenries inductor 128 26.667 millihenries in using these values, the center frequency of the circuit is 15.915 kilohertz or, nominally, 16 kilohertz.

For circuit 210, typical values of the circuit elements are as follows:

inductor 214 80 millihenries capacitor 216 .0032 microfarads resistor 220 5,000 ohms resistor 222 5,000 ohms resistor 224 12,071 ohms capacitor 228 .0013 microfarads In using these elements, the center frequency of circuit 210 is 9.946 kilohertz or, nominally, 10 kilohertz.

1 claim:

1. A filter circuit comprising: a pair of circuit portions, each circuit portion having a pair of opposed ends; a current generator; means coupling the ends of each circuit portion across the current generator, whereby the circuit portions are in parallel with each other, said current generator adapted to provide an input current for said circuit portions, each circuit portion having first and second current-carrying elements in series with each other, the first element being coupled to one side of the current generator and the second element being coupled to the other side of the current generator, there being a third current-carrying ele ment for each circuit portion, respectively, each third element being coupled between said other side of said current generator and the junction between the respective first and second elements, whereby the current through the first element is the sum of the currents through the second and third elements, the structure of said first element being sufficient to cause the current therethrough to be at a first phase angle with respect to said input current, the structures of the second and third elements of each circuit portion being sufficient to cause the current through the second element to be at a phase angle of approximately 90 to the current through the third element and to cause the currents of the second elements of said circuit portions to have respective magnitudes and directions relative to those of said input current so as to cause the vector sum of the currents of said second elements to be greater than the magnitude of the input current.

2. A filter circuit as set forth in claim 1, wherein said circuit has a center frequency proportional to the magnitudes of the second elements of said circuit portions, said vector sum being greater than the magnitude of said input current when the frequency of the latter is within a predetermined range of values on each side of said center frequency, respectively.

3. A filter circuit as set forth in claim 1, wherein the first and third elements of one of the circuit portions and the second element of the other circuit portion are resistors, the second element of said one circuit portion and the first and third elements of said other circuit portions being capacitors.

4. A filter circuit as set forth in claim 1, wherein said first and third elements of one of the circuit portions and the second element of the other circuit portion are resistors, the second element of said one circuit portion and the first and third elements of said other circuit portion being inductors.

5. A filter circuit as set forth in claim 1, wherein the first element of one of the circuit portions is an inductor, the second element of said one circuit portion and a third element of the other circuit portion being capacitors, the third element of said one circuit portion and the first and second elements of said other circuit portion being resistors.

6. A filter circuit having a center frequency adapted to supply a current having a predetermined frequency comprising: a current generator; a pair of circuit portions coupled across said current generator, each circuit portion including a first current-carrying element coupled at one side of said current generator and second and third current-carrying elements coupled between the first element and the other side of said our rent generator, the second and third elements being in parallel with each other, whereby the current from the current generator will divide and flow into said circuit portions and the current through the first element of each circuit portion will divide and flow through respective second and third elements thereof, the first element of each circuit portion having a structure sufficient to cause the current therethrough to be at a predetermined phase angle with respect to said generator current, the second and third elements of each circuit portion having structures sufficient to cause the currents therethrough to be at a phase angle of approximately to each other, the magnitudes and phase angles of the currents of the second elements of said circuit portions being sufficient to cause the vector sum of the currents through the second elements to be greater than the magnitude of the generator current when the ratio of the frequency of said generator current and said center frequency is a preselected value.

7. A filter circuit as set forth in claim 6, wherein said vector sum is greater than said generator current for ratios of said frequencies on each side of the ratio of unity, respectively.

8. A filter circuit as set forth in claim 6, wherein said third element of each circuit portion has a structure sufficient to cause the current flowing therethrough to have a value which is negative with respect to the current flowing through the corresponding second element.

9. A filter circuit as set forth in claim 6, wherein the phase angle between said vector sum and said generator current is zero when said ratio is substantially unity and has predetermined values for values of said ratio greater or less than unity.

10. in a filter circuit having a center frequency and provided with a current generator for supplying a current at a predetermined frequency: a pair of circuit portions, each circuit portion including a first currentcarrying element adapted to be coupled at one side of said current generator and second and third currentcarrying elements coupled to the first element and adapted to be coupled to the other side of said current generator, the second and third elements being in parallel with each other, whereby the current from the current generator will divide and flow into said circuit portions and the current through the first element of each circuit portion will divide and flow through respective second and third elements thereof, the first element of each circuit portion having a structure sufficient to cause the current therethrough to be at a predetermined phase angle with respect to said generator current, the second and third elements of each circuit portion having structures sufficient to cause the currents therethrough to be at a phase angle of approximately to each other, the magnitudes and phase angles of the currents of the second elements of said circuit portions being sufficient to cause the vector sum of the currents through the second elements to be greater than the magnitude of the generator current when the ratio of the frequency of said generator current and said center frequency is a preselected value. 

1. A filter circuit comprising: a pair of circuit portions, each circuit portion having a pair of opposed ends; a current generator; means coupling the ends of each circuit portion across the current generator, whereby the circuit portions are in parallel with each other, said current generator adapted to provide an input current for said circuit portions, each circuit portion having first and second current-carrying elements in series with each other, the first element being coupled to one side of the current generator and the second element being coupled to the other side of the current generator, there being a third current-carrying element for each circuit portion, respectively, each third element being coupled between said other side of said current generator and the junction between the respective first and second elements, whereby the current through the first element is the sum of the currents through the second and third elements, the structure of said first element being sufficient to cause the current therethrough to be at a first phase angle with respect to said input current, the structures of the second and third elements of each circuit portion being sufficient to cause the current through the second element to be at a phase angle of approximately 90* to the current through the third element and to cause the currents of the second elements of said circuit portions to have respective magnitudes and directions relative to those of said input current so as to cause the vector sum of the currents of said second elements to be greater than the magnitude of the input current.
 2. A filter circuit as set forth in claim 1, wherein said circuit has a center frequency proportional to the magnitudes of the second elements of said circuit portions, said vector sum being greater than the magnitude of said input current when the frequency of the latter is within a predetermined range of values on each side of said center frequency, respectively.
 3. A filter circuit as set forth in claim 1, wherein the first and third elements of one of the circuit portions and the second element of the other circuit portion are resistors, the second element of said one circuit portion and the first and third elements of said other circuit portions being capacitors.
 4. A filter circuit as set forth in claim 1, wherein said first and third elements of one of the circuit portions and the second element of the other circuit portion are resistors, the second element of said one circuit portion and the first and third elements of said other circuit portion being indUctors.
 5. A filter circuit as set forth in claim 1, wherein the first element of one of the circuit portions is an inductor, the second element of said one circuit portion and a third element of the other circuit portion being capacitors, the third element of said one circuit portion and the first and second elements of said other circuit portion being resistors.
 6. A filter circuit having a center frequency adapted to supply a current having a predetermined frequency comprising: a current generator; a pair of circuit portions coupled across said current generator, each circuit portion including a first current-carrying element coupled at one side of said current generator and second and third current-carrying elements coupled between the first element and the other side of said current generator, the second and third elements being in parallel with each other, whereby the current from the current generator will divide and flow into said circuit portions and the current through the first element of each circuit portion will divide and flow through respective second and third elements thereof, the first element of each circuit portion having a structure sufficient to cause the current therethrough to be at a predetermined phase angle with respect to said generator current, the second and third elements of each circuit portion having structures sufficient to cause the currents therethrough to be at a phase angle of approximately 90* to each other, the magnitudes and phase angles of the currents of the second elements of said circuit portions being sufficient to cause the vector sum of the currents through the second elements to be greater than the magnitude of the generator current when the ratio of the frequency of said generator current and said center frequency is a preselected value.
 7. A filter circuit as set forth in claim 6, wherein said vector sum is greater than said generator current for ratios of said frequencies on each side of the ratio of unity, respectively.
 8. A filter circuit as set forth in claim 6, wherein said third element of each circuit portion has a structure sufficient to cause the current flowing therethrough to have a value which is negative with respect to the current flowing through the corresponding second element.
 9. A filter circuit as set forth in claim 6, wherein the phase angle between said vector sum and said generator current is zero when said ratio is substantially unity and has predetermined values for values of said ratio greater or less than unity.
 10. In a filter circuit having a center frequency and provided with a current generator for supplying a current at a predetermined frequency: a pair of circuit portions, each circuit portion including a first current-carrying element adapted to be coupled at one side of said current generator and second and third current-carrying elements coupled to the first element and adapted to be coupled to the other side of said current generator, the second and third elements being in parallel with each other, whereby the current from the current generator will divide and flow into said circuit portions and the current through the first element of each circuit portion will divide and flow through respective second and third elements thereof, the first element of each circuit portion having a structure sufficient to cause the current therethrough to be at a predetermined phase angle with respect to said generator current, the second and third elements of each circuit portion having structures sufficient to cause the currents therethrough to be at a phase angle of approximately 90* to each other, the magnitudes and phase angles of the currents of the second elements of said circuit portions being sufficient to cause the vector sum of the currents through the second elements to be greater than the magnitude of the generator current when the ratio of the frequency of said generator current and said center frequency is a preselected value. 